The present invention relates generally to sulfonated polymer compositions that are suitable in particular for producing polymer electrolyte membranes, electrodes and membrane electrode assemblies for use in fuel cells, in electrolysis cells, in dialysis equipment and in ultrafiltration and methods of synthesizing polymer compositions. More specifically, the present invention relates to innovative crosslinked sulfonated poly(phenylene) copolymers, methods of making the same, and their use as a proton exchange membrane (PEM) in hydrogen fuel cells, direct methanol fuel cell, in electrode casting solutions and electrodes, and in hybrid sulfur electrolyzers.
Polymer electrolyte fuel cells (PEFCs) have great potential as an environmentally friendly energy source. Fuel cells are electrochemical energy converters which feature in particular a high level of efficiency. Among the various types of fuel cells, PEFCs feature high power density and a low weight to power ratio. The PEFC uses as its electrolyte a polymer membrane.
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Fuel cells are attractive electrical power sources, due to their higher energy efficiency and environmental compatibility compared to the internal combustion engine. The most well-known fuel cells are those using a gaseous fuel (such as hydrogen) with a gaseous oxidant (usually pure oxygen or atmospheric oxygen), and those fuel cells using direct feed organic fuels such as methanol.
The polymer electrolyte membrane or proton exchange membrane (PEM) is an important aspect of any PEFC. PEMs are an excellent conductor of hydrogen ions. The most widely used materials to date consist of a fluorocarbon polymer backbone, similar to Teflon®, to which are attached sulfonic acid groups. The acid molecules are fixed to the polymer and cannot “leak” out, but the protons on these acid groups are free to migrate through the membrane. With the solid polymer electrolyte, electrolyte loss is not an issue with regard to stack life. The potential power generated by a fuel cell stack depends on the number and size of the individual fuel cells that comprise the stack, and the surface area of the PEM.
In many fuel cells, the anode and/or cathode comprise a layer of electrically conductive, catalytically active particles (usually in a polymeric binder). A polymer electrolyte membrane is sandwiched between an anode and cathode, and the three components are sealed together to produce a single membrane electrode assembly (MEA). The anode and cathode are prepared by applying a small amount of a catalyst, for example, platinum (Pt) or ruthenium-platinum (Ru/Pt), in a polymeric binding to a surface that will be in contact with the PEM. Preparation of catalyst electrodes has traditionally been achieved by preparing an ink consisting of an electrocatalyst (either Pt or Ru/Pt), Nafion® polymer (5% wt. solution dispersed in lower alcohol). The ink is applied to porous carbon paper using a painting technique, or directly depositing the ink upon the membrane surface, or pressing it upon the membrane like a decal.
A MEA of a hydrogen fuel cell typically accepts hydrogen from a fuel gas stream that is consumed at the anode, yielding electrons to the anode and producing hydrogen ions, which enter the electrolyte. The polymer electrolyte membrane allows only the hydrogen ions to pass through it to the cathode while the electrons must travel along an external circuit to the cathode thereby creating an electrical current. At the cathode, oxygen combines with electrons from the cathode and hydrogen ions from the electrolyte to produce water. The water does not dissolve in the electrolyte and is, instead, rejected from the back of the cathode into the oxidant gas stream.
For the last 30 years the industry standard for the PEM component of a hydrogen or methanol fuel cell has been membranes based on fluorine-containing polymers, for example, the Nafion® material marketed by DuPont. Nafion® material is a perfluorinated sulfonic acid polymer having a well-known structure. Nafion® is often used as a membrane material for fuel cells, which operate at temperatures close to ambient. Further, Nafion® polymer membranes are hydrated and they have a hydrogen ionic conductivity of about 10−2S/cm or higher.
The Nafion® membranes display adequate proton conductivity, chemical resistance, and mechanical strength. Some of the membrane's disadvantages are reduced conductivity at high temperatures (>80° C.), high methanol permeability in direct methanol fuel cells, relatively thick membranes, and membrane dehydration at high temperatures. Further, when Nafion® membranes are used at temperatures above 80° C., they thermally deform. This deformation of the membrane prevents the Nafion® membrane from coming into sufficient contact with the electrode, thereby reducing fuel cell performance. Additionally, there is a need to reduce the costs associated with such membranes.
Another limitation of Nafion® membranes occurs in applications in methanol fuel cells. Nafion® membranes are permeable to methanol. Methanol crossover is inversely proportional to membrane thickness. Direct transport of the fuel (i.e. methanol) across the membrane to the cathode results in losses in efficiency. Increasing the membrane thickness results in decreased methanol crossover. However, thicker membranes result in increased Ohmic losses and decreased fuel cell performance.
Membranes that decrease the rate of methanol crossover would allow the use of higher concentrations of methanol-water feed mixtures, which would increase catalyst efficiency, direct methanol fuel cell power output, and potentially fuel utilization.
In general, increasing the operation temperature of fuel cells is advantageous for several reasons. Higher operating temperatures in methanol fuel cells decrease the carbon monoxide poisoning of the electrocatalyst. Higher temperatures increase reaction kinetics of hydrogen oxidation on the anode and oxygen reduction on the cathode. However, as the temperature is increased, it becomes more difficult to keep the membrane hydrated. Dehydration of membranes is exacerbated by relatively thick membranes. Dehydrated membranes lose ionic conductivity and result in poor contact between fuel cell components due to shrinkage of the membrane. Therefore, improved performance of fuel cells could be achieved by reducing the thickness of membranes, and improving the humidification state of solid PEMs, since water molecules can promote proton transport and thin membranes can reduce ionic resistance and Ohmic losses.
Additionally, the contact between the membrane and electrode affects the efficiency of a fuel cell. Interfacial resistance between the membrane and electrode causes Ohmic loss thereby decreasing fuel cell efficiency. Improving the membrane-electrode contact and continuity, wherein the membrane and electrode are cast from a composition having the same or similar polymer electrolytes, would improve the membrane-electrode interfacial resistance.
What is needed are compositions from which improved polymer electrolyte membranes, electrodes, and electrode casting solutions can be made that have improved performance at temperatures at about 80° C. and above, and preferably above 120° C. Operating at these temperatures results in enhanced diffusion rates and reaction kinetics for methanol oxidation, oxygen reduction, and CO desorption thereby producing a more efficient fuel cell.
PEMs are also used in electrochemical reactors that generate hydrogen gas by the following reaction:SO2+2H2O+electric power→H2SO4+H2+heat  (1)
This reaction can be part of the hybrid-sulfur thermochemical cycle in which the net reaction is the splitting of water. Reaction 1 can be performed in a membrane electrode assembly (i.e., electrolyzer), such as shown schematically in FIG. 1. Desirable features for an improved PEM material in such an electrolyzer application include: a) lower SO2 crossover rates and/or higher conductivity, b) efficient water transport, and c) high temperature operation (e.g., ≧120° C.). These improved characteristics should help to achieve the goal of reducing cell overvoltage (enabling higher efficiency), lowering SO2 crossover (for improved efficiency, and potentially durability), and providing more physically robust membrane structures to allow higher temperature operation if needed. PEMs that can readily operate above the melting point of sulfur (113-114° C.) could provide a method for sulfur removal, as well as enhance the kinetics of reaction at the electrolytic cell electrodes.
Previous sulfonated PEM membranes were synthesized at Sandia National Laboratories (SNL) for the DOE fuel cell program, and later patented (U.S. Pat. No. 7,301,002, which is incorporated herein by reference). These previous membrane materials, an example of which is shown in FIG. 2, were composed of a sulfonated poly(phenylene) compound that was prepared by a Diels-Alder reaction; and which will hereafter be referred to by the acronym SDAPP. The polymerization reaction to make the unsulfonated parent polymer is an irreversible Diels-Alder reaction that is responsible for forming every other phenyl ring in the backbone. Due to the ambiguous regiochemistry of the reaction, a mixture of 1,4 and 1,3-substituted rings are formed. The parent polymer is treated with a sulfonating agent to put sulfonic acid groups on the para-positions of some of the pendant phenyl rings (R=SO3H in FIG. 2). The number of sulfonic acid groups formed, and thus the ion exchange capacity (IEC), can be controlled by varying the amount of sulfonating agent used.
These previous SDAPP membranes were tested at SNL in an electrolysis cell as an alternative to Nafion® in a Hybrid-Sulfur (HyS) electrolyzer application. That test data, shown in FIG. 3, showed that these membranes could operate near 120° C., with somewhat reduced voltages (˜0.7 volts), at a current density of about 0.43 amps/cm2. For those tests, the cell conditions were: cell size=10 cm2, cell potential=0.7 V, SDAPP 2.2 meq/g membrane batch, 2 mg Pt/cm2 Pt Black anode and cathode, Dry SO2 gas anode, 100 sccm constant SO2 flow rate with 15 psig backpressure, and preheated liquid water cathode, 3 mL/min constant water flow rate with 15 psig backpressure.
The SDAPP family of membranes afford advantages over Nafion® membranes in temperature and transport capabilities. Based on previous SNL data, Nafion® membranes have approximately twice the undesirable SO2 crossover rate (see FIG. 4) and generate nominally 30% lower outlet acid concentration, which could be a key process variable. FIG. 5 shows the SO2 crossover effect measured in the SNL electrolysis cell for a 60 micron thick SDAPP membrane (SDAPP 2.2), and a 90 micron thick Nafion® 212 membrane (Lynntech MEA), as a function of temperature. The undesirable SO2 crossover effect results in: a) process loss, b) parasitic H2 consumption, and c) elemental sulfur buildup on the cathode, which blocks reaction sites. Note in FIG. 5 that the SO2 flux to the cathode is lower for the SDAPP membrane, even though the SDAPP membrane is thinner.
Previous tests with SDAPP in the SNL hybrid-sulfur electrolyzer in 2007 showed that the cell current increased as the cell temperature was raised (at a constant voltage) from 80 to 120° C. As the temperature was raised above 120° C., however, the current began to decrease, presumably due to an increase in the resistance of the membrane. This increase in membrane resistance is believed to be due to a change in the membrane morphology. At lower temperatures (≦120° C.), the polymer chains that compose the membrane are phase separated into microdomains that are either hydrophilic or hydrophobic. The size and interconnectivity of the hydrophilic domains determine the water swelling and ionic conductivity of the membrane. At temperatures above 120° C., the microstructure undergoes a change and the ion-conducting channels within the membrane are disrupted, thereby increasing resistance.
Therefore, the approach taken in the present invention was to crosslink the polymers in order to “lock in” the desired morphology and prevent it from changing at higher temperatures. Crosslinking limits the ability of the membranes to swell, and the lower water content could reduce the ionic conductivity. Crosslinking will also allow for the density of ionic functional groups (sulfonic acids in this case) to be increased (higher IEC) without causing the membranes to swell too much or even become soluble in water. The increased IEC values could lead to membranes with higher conductivities than those for the membranes that are not crosslinked, despite their lower water swelling.